1. Field of the Invention
This invention relates to dynamic magnetic information storage and retrieval devices, specifically to the positioning of a read/write transducer used within such devices.
2. Discussion of Prior Art
Dynamic magnetic information storage and retrieval devices of the type commonly referred to as hard disk drives can be composed of rotating flat magnetic disks with scanning transducers which read and write data in the form of magnetic transitions in concentric circular recording tracks on the disks. The transducers, mounted to flat sliders, "fly" a microscopically small distance above the surface of the respective disk on a thin, self-pressurizing bearing of ambient atmospheric air.
During the manufacturing process of a disk drive it is desirable to determine the electromagnetic performance of a transducer as it flies above a written magnetic track on a disk. It is desirable that this be done both as it is held in perfect alignment with a respective track and as it is held a small known radial distance away from alignment, on the order of 0.025 to 25 microns, simulating operation in a typical imperfect environment.
Heretofore, in a transducer or disk or disk drive test system, elaborate and expensive means were involved in moving the transducer a small known distance from perfect alignment with a respective track.
Test systems containing mechanisms consisting of precision linear ball- or roller-bearing slides and leadscrews have been used for positioning the transducer. These devices suffer from a problem known in the art as "stick-slip", whereby the actuation force in the positioning system increases until it is just sufficient to overcome friction. As friction is overcome the carriage of the positioning system suddenly moves, typically in an oscillatory manner, limited by the interaction between the compliance of the actuation means, the mass of the carriage and the friction opposing carriage motion. This phenomenon creates a limit to the minimum size that a positioning system can reliably move, such size being greater than desirable in even the most precise mechanisms of this kind.
Such devices also contain hysteresis such that movements in opposite directions create positioning errors. Compliance of the structural members within the system causes deformation when acted upon by the frictional forces. Since the frictional force acts in the direction opposing motion, this deformation is not the same for motions in opposite directions, resulting in positioning inaccuracies.
Mechanisms consisting of air bearing slides eliminate the friction and hysteresis problems due to the elimination of contact between moving surfaces by the air film, but are prohibitively expensive and require pressurized, filtered, dried air. These air compression and filtration systems are large, bulky and expensive. They require significant effort to install and maintain. High pressure air lines must be installed from the air compression system to the transducer test system, increasing the effort required to install or relocate the transducer test system.
Mechanisms employing some type of flexure pivot device as a secondary positioner on a primary gross positioner have been used to eliminate the friction and hysteresis of the transducer support means. All such mechanisms heretofore known have had the secondary positioner driven directly by some type of actuation means whereby the resolution and accuracy of the transducer positioning is limited to approximately that of the actuation means. Actuation means having sufficient resolution and accuracy, on the order of 0.025 micron, are prohibitively expensive.
Movement devices employing the electrostriction of piezoelectric crystals have also been utilized as secondary positioners. However, they suffer from a problem known in the art as creep, whereby the crystal continues electrostricting long after the driving signal has been applied, making accurate control of position difficult to achieve. These devices also require a high electrical voltage for operation, which requires additional means to ensure operator safety and adds considerably to the cost of a system. Additionally these devices require that the electrical input voltage remain applied for the crystal to remain expanded. This may be incompatible with the low-level electrical signals which are being measured from the transducer. Such devices also exhibit poor repeatability, whereby a given applied voltage produces a different movement size from one day to the next, making controlled accurate motion difficult.
All of the positioning means heretofore known suffer from one or more of the following disadvantages:
(a) The resolution of the transducer motion is limited to that of the positioning actuator. Positioning actuators with sufficient resolution, approximately 0.025 micron, are prohibitively expensive.
(b) The positional errors and inaccuracies of the transducer positioning actuator appear at the transducer on an approximately one-for-one basis. The addition of heretofore used mechanisms to reduce errors has not been successful, either in feasibility or cost effectiveness.
(c) Friction between moving parts in the positioning apparatus prevents the successful implementation of very small incremental movements. Means to eliminate the friction, such as the use of air bearings, are prohibitively expensive.
(d) Hysteresis in the positioning apparatus causes bidirectional positioning to be inaccurate. Means of eliminating the hysteresis are prohibitively expensive.
(e) The various subsystems must be carefully aligned for operation to be successful and accurate. The alignment of the transducer to the disk is dependent on these subsystem alignments and must be carefully controlled. Significant manufacturing time must be spent on this alignment procedure. The manufacturing tolerances on individual parts must be quite small which increases their costs significantly.
(f) The sizes of typical transducer test systems are large due to the required subsystems and their ancillary equipment. The large required space for these systems adds significantly to the cost of the entire manufacturing operation.
(g) The moving elements of the transducer positioning system have limited life due to wear of interfaces between moving parts. Because usage of a typical test system in a manufacturing situation is high, wear can occur quickly. Because tolerance for misalignment is low a small amount of wear can render a system useless.
(h) The moving elements of the transducer positioning system require lubrication for operation. The lubricant, upon migration, can cause contamination of the transducer's slider to disk interface unless it is carefully sealed inside the moving element. The sealing system adds expense and complexity to the overall cost.
(i) Vibration of the transducer causes inaccurate positioning and measurement errors. This vibration, induced by imbalance in the rotating disk spindle or other sources, can be amplified by mechanical resonances in the transducer support structure.
(j) The transducer positioning system is heavy in weight such that rapid positioning of the transducer requires significant power. This in turn requires the use of larger, more expensive motors and amplifiers, as well as more input power.
(k) The transducer positioning system is structurally compliant such that a rapid positioning movement of the transducer causes structural oscillations to occur in the system itself and in the supporting structure. This in turn requires additional settling time which in turn reduces throughput and increases the cost to manufacture the transducer, disk or disk drive.